1. Field of the Invention
The present invention relates to an actuator for an optical pickup apparatus, and in particular to an improved actuator for an optical pickup apparatus which is capable of increasing the efficiency of a focusing coil and tracking coil by arranging the focusing coil and tracking coil in the same plane and disposing more than at least one permanent magnet in order for a plurality of poles to be alternately generated thereby, for thus improving the efficiency of an optical pickup apparatus for a high density optical disc which has been made to be operated at a high speed.
2. Description of the Background Art
Generally, the actuator is a device for performing focusing and tracking operations for an optical pickup apparatus.
In addition, the focusing operation is directed to focusing a beam onto anhoptiical disc surface by upwardly and downwardly moving an objective lens and maintaining a predetermined distance between the objective lens and the optical disc, and the tracking operation is directed to urging the beam passed through the objective lens to track the optical disc by leftwardly and rightwardly moving the objective lens.
Generally, many methods use electric magnetic forces generated between the permanent magnet and coils for focusing and tracking operations based on the objective lens.
FIG. 1 is a perspectives view illustrating the construction of a conventional actuator, FIG. 2A is a cross-sectional view taken along line IIa--IIa of FIG. 1, FIG. 2B is a cross-sectional view taken along line IIb--lIb of FIG. 2A, FIG. 2C is a cross-sectional view taken along line IIc--IIc of FIG. 2A, and FIG. 2D is a cross-sectional view taken along line IId--IId. of FIG. 2A.
As shown therein, a frame 2 is disposed in a portion of the upper surface of a plate-shaped yoke 1. End portions of two wires 4 are internally connected to a portion of the frame 2.
Other end portions of the two wires 4 are connected to a bobbin 5 having an objective lens 6.
In addition, back yokes 9a are spaced-apart with respect to the bobbin 5 on the yoke 1.
Permanent magnets 10 and 10' are attached to each lateral surface of the back yokes 9 and 9a the lateral surfaces of which are opposite to the bobbin 5. The focussing coil 7 is wound onto the bobbin 5.
Tracking coils 8L, 8R and 8L', 8R' are arranged opposite to the permanent magnets 10 and 10' in both shorter sides of the focusing coil 7 being opposite to the permanent magnets 10 and 10', and the tracking coils BL, 8R and 8L', 8R, are rectangularly wound thereonto.
In addition, inner back yokes 11 and 11a, which are integral with the yoke, are disposed between the focusing coil 7 to which the tracking coils 8L, 8R are attached and the bobbin 5.
Namely, the conventional actuator for an optical pickup apparatus basically includes a driving section having the bobbin, the focusing coil 7 disposed in the bobbin 5, and the tracking coils 8L, 8R, and 8L', 8R'.
The permanent magnets 10 and 10' attached to the back yokes 5 and 9a, as shown in FIGS. 2A and 2B, are arranged in order for the N-pole to be opposite to the bobbin 5, and the magnetic fluxes from the permanent magnets 10 and 10' pass through the tracking coils 8L, 8R and 8L', 8R' , the focusing coil 7, and the inner back yoke 11 and 11a, for thus forming a magnetic circuit.
In the drawings, reference:numeral 3 denotes a substrate.
The operation of the conventlonal actuator for an optical pickup apparatus will now be explained.
The operation for adjusting the distance between the objective lens 6 and the optical disc by upwardly and downwardly moving the bobbin 5 will now be explained.
First, since the magnetic fluxes generated by the permanent magnets 10 and 10' vertically flow with respect to the lengthy side of the focusing coil 7, the bobbin 6 is moved in the +Z direction or -Z direction as the focusing coil 7, as shown in FIGS. 2A and 2B, receives the moving force in the +z direction and -Z direction based n the flowing direction of the current applied to the focusing coil 7.
Therefore, the objectiive lens 6 disposed in the bobbin 5 is moved in the Z direction, for thus focusing the beam.
The principle oft the tracking operation will now be explained.
The current flowing directions of two tracking coils 8L, 8R being opposite to the permanent magnet 10 are opposite.
Therefore, the current flowing directions at a right side of the left side tracking coil 8L and a left side of the right side tracking coil 8R, as shown in FIG. 2D, are the same, so that the tracking coils 8L, 8R receive a force in the Y-axis direction as shown in FIG. 2B.
The tracking coils 8L and 8R receiving the above-described forces are integrally moved together with the bobbin 5 When the force is applied to the tracking coils 8L, 8R, the objective lens 6 is leftwardly and rightwardly moved by the movement of the bobbin 5, for thus pertforming the tracking operation.
The movement of the tracking coils 8L', 8R' is performed in the above-described manner as well.
However, the usage efficiency of the tracking coils 8L, 8R and 8L', 8R' are low in the conventional actuator for an optical pickup apparatus.
Namely, since there are only a small right side portion of the left side tracking coil 8L and a small left side portion of the right side tracking coil 8R which portions are actually used for the tracking operation of the tracking coils 8L and 8R, the remaining portions of the tracking coils 8L, 8R are not used for the tracking operation.
In addition, since the tracking coils 8L, 8R are overlapped on the upper surface of the focusing coil 7, the length of an efffective air gap may be made longer, for thus decreasing the average magnetic flux density of the coils.
Furthermore, in the conventional actuator for an optical pickup apparatus, since the magnetic flux lines FL are formed as shown in FIG. 2A, the inner back yokes 11 and 11a are necessary. Since the above-described inner back yokes 11 and 11a are fixedly used, it is necessary to form an air gap between the bobbin 5, which is a driving section, and the focusing coil 7, so that the bobbin 5 is made bulky, and the length of the focusing coil 7 is made longer.